The present invention relates to a kiln, furnace or the like and more particularly to such a furnace as may be suitable for use by dental laboratories in the firing of dental porcelain or crowns, bridges and the like.
It is desirable to fire dental porcelain in a subatmospheric environment. Accordingly, dental porcelain firing furnaces, as are known in the art, have a muffle housed within a chamber that can be evacuated. Since this chamber must be air tight, the replacement of burned out muffles is usually a tedious process resulting in considerable down time of the furnace.
It is also known in the art to have such furnaces bottom loaded. In this way, the raw crown or bridge work can be gradually raised into the furnace muffle so that the heat of the furnace will dry and preheat the workpiece prior to firing. Bottom loading has proved to be more heat efficient under these circumstances in that less heat escapes the muffle than from a front loading door.
A bottom loading furnace permits a more uniform distribution of heat about the work piece whereas a front loading furnace for example, may have a cold spot at the front. However, there must be a means to seal the bottom loading door against the muffle in order to draw a vacuum within the chamber. In the prior art, this has been accomplished by an O-ring attached to the door. Such O-rings have had a short life in that with the door partly open, and in a position to dry the restoration prior to firing, the seal on the door is directly exposed to the heat of the furnace.
In contrast to the prior art, the vacuum firing porcelain furnace of the present invention is easily assembled or disassembled thereby greatly facilitating muffle replacement. The present invention also relocates the seal means for the door of the furnace which increases seal life.